In recent years, to cope with the problem of global warming and promote effective use of energy resources, efforts to reduce energy consumption have been made in various fields. In the filed of illumination, known incandescent lamps are being shifted to fluorescent lamps with higher energy efficiency, and recently the latter has become widely used.
However, there has been a problem in replacing an incandescent lamp with a fluorescent lamp. That is, an economical incandescent lamp lighting fixture also has to be changed to an expensive lighting fixture with a built-in ballast for operating a fluorescent lamp.
To solve this problem, an incandescent-lamp-substituting fluorescent lamp which can be directly connected to an incandescent lamp socket in an incandescent lamp lighting fixture and includes a base and a ballast has been developed. The incandescent-lamp-substituting fluorescent lamp can be used with an incandescent lamp lighting fixture in place of an incandescent lamp and consumes less power. Moreover, the lifetime of the incandescent-lamp-substituting fluorescent lamp is over three times as long as that of an incandescent lamp. For the above-described reasons, the incandescent-lamp-substituting fluorescent lamp has now been widely used.
On the other hand, for the purpose of further increasing the lifetime of fluorescent lamps, there has been developed an electrodeless fluorescent lamp in which no electrode, causing loss of the lifetime of a fluorescent lamp, is provided. In the electrodeless fluorescent lamp, a high-frequency alternating electromagnetic filed is applied from the outside to a closed glass discharge vessel in which a noble gas and mercury are enclosed and a luminophor is applied to the inside wall, so that mercury vapor discharge is generated within the discharge vessel. Thus, ultraviolet radiation resulting from the mercury vapor discharge excites the luminophor to make it emit light. In this manner, the electrodeless fluorescent lamp is based on a different light emitting principle to the principle on which known fluorescent lamp including an electrode is operated. With the electrodeless fluorescent lamp, it is possible to achieve lifetime over twice as long as that of a known electrode-included fluorescent lamp.
As an electrodeless fluorescent lamp, an electrodeless fluorescent lamp including a base, a coil for generating a high-frequency alternating electromagnetic filed, a ballast circuit through which an alternating current flows, and the discharge vessel which does not include an electrode has been developed for the purpose of providing substitutes for incandescent lamps.
Such an incandescent-lamp-substituting electrodeless fluorescent lamp (which will be hereinafter referred to as an “electrodeless compact self-ballasted fluorescent lamp”) is assumed to be connected to an incandescent lamp lighting fixture. Thus, the electrodeless compact self-ballasted fluorescent lamp is required to have substantially the same shape and size as those of incandescent lamps. Recently, an electrodeless compact self-ballasted fluorescent lamp having close shape and size to those of incandescent lamps has been achieved.
Problems to be Solved
However, an incandescent lamp and an electrodeless compact self-ballasted fluorescent lamp are based on different light emitting principles. Therefore, luminous intensity distributions are different between the incandescent lamp and the electrodeless compact self-ballasted fluorescent lamp. Their respective luminous intensity distributions are shown in FIGS. 4 and 5. FIG. 4 is a graph showing characteristics of the luminous intensity distribution of a 60-watt silica incandescent lamp having an A type shape. FIG. 5 is a graph showing characteristics of the luminous intensity. distribution of a known electrodeless compact self-ballasted fluorescent lamp having also an A type shape. In each of FIGS. 4 and 5, the characteristics of the luminous intensity distribution of the incandescent lamp or the electrodeless compact self-ballasted fluorescent lamp when the lamp is placed with its base up are shown, and the upper side is the base side. Herein, the A type shape is a shape defined in JIS C7710-1988: Designation Method for Glass Bulbs of Lamps or in IEC 60887-1988. Note that IEC is an abbreviation of International Electrotechnical Commission.
Hereinafter, the respective light emitting principles on which the incandescent lamp and the electrodeless compact self-ballasted fluorescent lamp are based and differences between the respective luminous intensity distribution characteristics of the incandescent lamp and the known electrodeless compact self-ballasted fluorescent lamp due to differences between the principles on which the lamps are based will be described.
First, the respective light emitting principles on which the incandescent lamp and the electrodeless fluorescent lamp are based will be described.
In the case of the silica incandescent lamp, red heat irradiation from a filament located in the center of the lamp is diffused by a silica film applied to an outer tube.
On the other hand, the principle on which the known electrodeless compact self-ballasted fluorescent lamp is based is closely related to the structure of the lamp. Therefore, the principle used for the known electrodeless compact self-ballasted fluorescent lamp will be described as well as the structure of the known electrodeless compact self-ballasted fluorescent lamp shown in FIG. 8.
An A type shaped discharge vessel 11 made of soda glass includes an outer tube 31 and an inner tube 32 in which a cavity portion 12 having an approximately cylindrical shape is defined. In the cavity portion 12, a core 14 made of ferrite is disposed. Around the core 14, a coil 13 for generating an alternating electromagnetic field in the discharge vessel 11 is wound. A plasma 15 is generated by the generated alternating electromagnetic field. In this manner, the coil 13 and the core 14 are disposed to generate an alternating electromagnetic field and thereby the plasma 15 is generated in a ring shape so as to surround the coil 13 and the core 14 in the discharge vessel 11. An ultraviolet light generated by a discharge of the plasma 15 excites a luminophor film 16 evenly applied to the inside wall of the discharge vessel 11 to make the luminophor film 16 emit light. In this manner, visible light is generated. Note that the coil 13 is electrically connected to a ballast circuit 17 for supplying an alternating current to the coil 13, and then the ballast circuit 17 is electrically connected to a base 18 to be connected to the commercial power line. Moreover, a case 19 is provided so as to surround the ballast circuit 17, and the discharge vessel 11 and the base 18 are attached to the case 19. Note that for the purpose of simplification, the cross-section of each of the discharge vessel 11, the cavity portion 12 and the case 19 is indicated as a line.
Next, differences between the respective luminous intensity distribution characteristics of the incandescent lamp and the known electrodeless compact self-ballasted fluorescent lamp due to differences between the principles on which the lamps are based will be described.
As has been described, in the silica incandescent lamp, red heat irradiation from the filament located in the center of the lamp is diffused by the silica film applied to the outer tube. The light diffusion amount at the wall surface of the outer tube is small, and luminance is highest at a filament portion of the lamp. Moreover, the filament is located around the center of the curvature of the outer tube and the size of the filament is sufficiently smaller than the radius of the curvature. Thus, the silica incandescent lamp is considered to be a point light source whose center point is the filament. Accordingly, seen either from the side of the outer tube opposite to the base (i.e., an edge of the outer tube) or from the side face of the outer tube, the brightness of the tube seems almost the same. Therefore, as shown in FIG. 4, a substantially uniform luminous intensity distribution, except for vignetting of the base, is obtained. The luminous intensity distribution characteristics are the almost the same when the shape of the incandescent lamp is either an A type shape or a P type shape. Note that the P type shape is a shape defined in JIS C7710-1988: Designation Method for Glass Bulbs of Lamps or in IEC 60887-1988.
On the other hand, in the electrodeless fluorescent lamp, light is emitted out of the discharge vessel 11 of the electrodeless fluorescent lamp in the manner in which light emitted from the luminophor film 16 is repeatedly reflected inside of the of the discharge vessel 11 and part of the light transmits through the luminophor film 16. Because luminophor film 16 has a uniform thickness, the discharge vessel 11 is considered to be a light source having the entire surface with uniform luminance. In this manner, the electrodeless fluorescent lamp has uniform luminance at the entire surface, and thus the luminous intensity distribution is proportional to the apparent area of the surface. Accordingly, when the electrodeless compact self-ballasted fluorescent lamp having an A type shape and using the discharge vessel 11 is operated with its base up (in a base-up position), the apparent area of the lamp surface seen from directly under the lamp is smaller than that seen from the side (the lateral direction) and luminous intensity of light toward directly under the lamp is small, except for the case where the lamp seen from the base direction. The luminous intensity distribution characteristics have the same tendency as described above when the electrodeless compact self-ballasted fluorescent lamp has either an A type shape or a P type shape.
As in the description above, even if the silica incandescent lamp and the electrodeless fluorescent lamp have the same shape and size, the respective luminous intensity distributions of the lamps have different characteristics because the silica incandescent lamp and the electrodeless fluorescent lamp are based on different light-emitting principles.
Also, as an electrodeless reflector fluorescent lamp, which has a different shape from the A type shape and the P type shape, there has been studies of electrodeless fluorescent lamps in which a reflection film is provided in a region of the inner surface of an outer tube extending from the vicinity of a base to a portion of the outer tube having the maximum diameter (for example, see Japanese Unexamined Patent Publication No. 8-45481) or a reflector is provided in the same.
However, incandescent lamp lighting fixtures which have been widely used in present are designed so that light is taken out most efficiently when a lamp having the same luminous intensity distribution characteristics as those of an incandescent lamp is connected. Accordingly, even if the known electrodeless compact self-ballasted fluorescent lamp is connected to a widely-used lighting fixture, light can not be efficiently taken out because the electrodeless compact self-ballasted fluorescent lamp has different luminous intensity distribution characteristics from those of an incandescent lamp. In other respects than efficiency in taking light out, for example, when the electrodeless compact self-ballasted fluorescent lamp is connected to a lighting fixture located around the ceiling and used as a downlight, the tendency in which luminous intensity of light toward directly under the lamp is small as shown in FIG. 5 is further emphasized. As a result, an edge portion of the lamp unpreferably looks dark, compared to the periphery of the edge portion.
It can be another option to apply a material which absorbs light to part of the outer surface of the lamp in which the luminous intensity is high in order to control luminous intensity distribution characteristics, but the total luminous flux becomes small, resulting in reduction in efficiency. Therefore, this option is not practical.
Moreover, the electrodeless fluorescent lamp disclosed in the publication above does not have an incandescent lamp shape. Because of this difference in shape, the electrodeless fluorescent lamp can not be used as a substitute for an incandescent lamp. Furthermore, when the electrodeless fluorescent lamp is used with an incandescent table lamp to which the electrodeless fluorescent lamp can be connected with its base down, no light is taken out under the table lamp. Therefore, the electrodeless fluorescent lamp can not be used with such a table lamp with its base down (in a base-down position).
The present invention has been devised in view of the above-described problems and it is therefore an object of the present invention to provide an electrodeless fluorescent lamp which has approximately the same luminous intensity distribution characteristics as those of an incandescent lamp and is suited to an incandescent lamp lighting fixture.